What is Molecular Chemistry?

Polymer chemistry is an emerging comprehensive discipline that studies the synthesis, chemical reaction, physical chemistry, physics, processing molding, and application of polymer compounds.

Chemical molecule

synthesis

Humans actually have a close relationship with polymers from the beginning. Animals and plants in nature, including the human body, are made of polymers as the main component. These polymers have long been used as raw materials for manufacturing tools and means of living. Human's main foods such as starch and protein are also polymers. It was only after a large number of industrially synthesized macromolecules and important applications were obtained that these artificially synthesized compounds gained the name polymer compounds.

Later, after research, it was known that artificially synthesized polymers and those naturally occurring polymers have similarities in structure and performance, so they are called polymer compounds. Synthetic polymers synthesized in the industry or in the laboratory are called synthetic polymers. Generally speaking, most of the polymers refer to synthetic polymers. Naturally occurring polymers are referred to as natural polymers.

Practice has proven that many olefinic compounds can undergo chain reactions and rapidly form polymers after being initiated by organic free radicals. From the early 1930s to the early 1940s, many current general polymer varieties have been put into industrial production according to this method. In the late 1930s, Carothers discovered the use of polycondensation to synthesize polymers. Later, for reasonable processing and effective applications, research on the structure and properties of polymers has gradually been carried out, making polymers a widely used material. At the same time, a new comprehensive disciplinepolymer sciencehas flourished since the second half of the 1940s.

Polymer science can be divided into three parts: polymer chemistry (narrow sense), polymer physics, and polymer technology. Polymer chemistry is divided into polymer synthesis, polymer chemical reaction and polymer physical chemistry. Polymer physics studies the aggregate structure and bulk properties of polymers. Polymer technology is divided into polymer processing and polymer applications.

Although the molecular weight of the polymers is very high, the functional groups they possess still have the same reactivity as ordinary small molecule organic compounds. However, its reaction performance is affected by two unique factors: the polymer is a long chain structure, and this long chain is a zigzag shape. Regular buckling (folding) forms a crystalline state, and irregular buckling forms an amorphous state; the molecules of the polymer are stacked with the molecules. Regular stacking forms a regular crystalline arrangement; irregular stacking forms an amorphous state. In the regular structure, the molecules are tightly arranged, the reagent is not easy to invade, and the functional group is not easy to react; in the irregular structure, the molecular arrangement is loose, the reagent is easy to invade, and the functional group is easy to react.

It has been studied and used by people as early as the 19th century. In 1845 Schoenbein discovered that cellulose could be nitrated into nitrocellulose. In 1865 Hützenberger acetylated cellulose into cellulose acetate. The production of viscose rayon is also achieved through chemical changes in cellulose.

Some of the chemical reactions of polymers are destructive, such as photodegradation of polymers, thermal degradation of polymers, and oxidation of polymers. They age polymer materials,

The polymer chain structure is a primary structure; an isolated polymer chain, that is, the form of a polymer in a dilute solution, such as a random coil, a spiral, a double helix, a rigid rod or an ellipsoid, etc. is a secondary structure; Aggregate molecular structure, that is, the packing between molecular chains. The aggregate structure varies with the processing method. Polymers with aggregated structures are called high polymers.

Most linear polymers can be dissolved in the corresponding solvent to form a solution. The polymer solution is a true solution, not a colloidal solution previously thought. The polymer is a long-chain structure, which can block each other when flowing, so the polymer solution is viscous. In general, the longer the molecular chain, the greater the viscosity. When the light beam passes through the polymer solution, light scattering can occur because the polymer is relatively large. The larger the molecule, the stronger the scattering.

Polymers are much heavier than solvent molecules. Under ultra-high speed centrifugation, polymers move faster than solvent molecules and diffuse more slowly than solvent molecules. The larger the molecular weight, the more obvious these differences are. Using these polymer solution properties, the molecular weight of the polymer can be measured. When studying polymer solutions, in addition to measuring molecular weight and its distribution, the morphology and structure of polymers can be estimated from various properties of the solution.

Different from small molecules, polymers have strength, modulus, and mechanical properties such as viscoelasticity, fatigue, and relaxation. They also have optical, thermal, acoustic, and electrical physical properties such as light transmission, heat preservation, sound insulation, and resistance. Polymers can be used as a variety of materials. The structure of the polymer is related to the processing method. Therefore, in order to obtain the excellent properties of the polymer, it is necessary to adopt a suitable processing and molding method so that it forms a proper structure. For example, fiber-forming polymers must be oriented at a specific temperature after spinning to achieve higher strength.

Polymers are used as materials. They are mainly divided into plastics, fibers, and rubbers. All of them need to be processed into certain shapes before they can be used. In addition, ion exchange resins used as separation and analysis materials can be made into spherical particles during the polymerization process; high polymers used as paints and coatings can be used as long as they are dissolved in a suitable solvent without processing. forming.

The rapid development of polymer production illustrates the rapid increase in society's demand for it. High-molecular materials were first used as insulating materials, and the amount is still very large, especially new high-insulating materials. For example, polyester film is far superior to mica sheet; silicone paint is used as wire insulation paint, which is not comparable to gauze insulated wire. With the emergence of various new and excellent polymer dielectric materials, new technologies such as the electronics industry and computers and remote sensing can be established and developed.

Polymers are used as structural materials to replace wood, metal, ceramics, glass and other applications. In agriculture, industry and daily use, it has many advantages, such as light weight, non-corrosive, non-corrosive, colorful, etc. It is used for mechanical parts, vehicle and ship materials, industrial pipeline containers, agricultural films, packaging bottles, boxes, paper , Construction boards, pipes, rods, etc., are not only cheap and good, but also easy to assemble. It can also be used in medical equipment, household appliances, cultural, sports, entertainment products, children's toys, etc., greatly enriching and beautifying people's lives.

The superiority of synthetic fibers, such as soft, non-crepe, strong, stiff, not moldy, etc., is also beyond the natural fiber cotton, wool, silk, hemp and so on. It is especially important that they do not compete with food for land. The synthetic fiber produced by a factory can be a natural fiber that can be produced in millions of acres of farmland. The production of natural rubber is limited by the region, and the output cannot meet the increasing requirements. However, synthetic rubber is not subject to such restrictions, and each variety has advantages over natural rubber.

It is generally believed that polymer materials have low strength and poor heat resistance. This is the impression obtained from common plastics. Now the toughest material is not steel, not rhenium, not beryllium, but a reinforced plastic made of carbon fiber and epoxy resin. Heat-resistant polymer, can be used at 300 degrees Celsius for a long time.

It should be particularly mentioned that in space technology, when a rocket or satellite casing returns from the outer space to the atmosphere, the speed is high and the surface temperature can reach 5000 to 10,000 degrees Celsius. No natural material or metal material can withstand this high temperature However, reinforced plastic can do the job, because it decomposes when exposed to heat, emits a large amount of volatile gases, and absorbs a large amount of thermal energy, so that the temperature will not be too high. At the same time, plastic does not transfer heat, and can still maintain the temperature required for the normal work and life of people and instruments inside the housing. Good ablative material, the outer layer is only damaged by 3-4 cm, and the interior can be preserved to complete the task of returning to the ground.

However, polymer materials also have many weaknesses that must be researched to overcome them. For example, it is easy to burn. When a large amount of polymer materials are used, fire prevention is a big problem. The polymer must be made difficult to burn before it can be used safely. It is easy to age and not last long. Used as a building material, it must have a life span of at least several decades; if it is used in other fields, it must also have durability.

When a large amount of polymer materials are used, the polymer garbage thrown away as waste will not be dissolved and weathered by water, and will not be corroded by bacteria. If it is not treated, it will accumulate more and become a serious public hazard. It is necessary to try to make polymer materials decompose and disappear in time after use.